Nanocomposites of thermoplastic polymers and/or elastomers which contain a dispersion of particles of intercalated, and possibly partially exfoliated, clay have heretofore been prepared, for example, by pre-intercalating a multi-layered, hydrophilic water-swellable clay in water which contains an intercalating compound (e.g. a quaternary ammonium salt) to form an organoclay followed by drying the organoclay to form an organoclay powder. The organoclay is then mixed with an elastomer to form a dispersion thereof in the elastomer. To a small extent, the layers of the intercalated clay (the organoclay) may become delaminated, or exfoliated, into individual platelets, including delaminated stacks of platelets, either during the intercalation process or upon high shear mixing with the elastomer.
Such a method is considered herein to be excessively process dependent and relatively inefficient insofar as obtaining a dispersion of substantially exfoliated platelets of an intercalated water-swellable clay within an elastomer matrix and therefore likely to be excessively costly to implement in a rubber product manufacturing operation.
In order to enhance reinforcement of elastomer-based components of articles of manufacture, particularly tires and more particularly tire treads, it is considered herein that it is desirable to present the water-swellable clay in a maximized state of exfoliation (e.g. maximized individual platelet formation with a uniform distribution of said platelets within the elastomer composition).
Such an exfoliation process is contemplated by utilizing an ion exchange phenomenon between cation exchangeable ions contained within the galleries of stacked platelets of a water-swellable clay composed of multiple layers of negatively charged stacked platelets and cationically (positively) charged elastomer particles contained in an aqueous latex of the elastomer particles. By such method the exfoliated platelets are thereby created in situ.
The exfoliation process for this invention is conducted to the exclusion of a thermoplastic polymer latex and to the exclusion of more conventional anionic (negatively charged) elastomer particles derived from an elastomer latex stabilized with an anionic-type surfactant.
The exfoliation maximization is based, at least in part, upon a destabilization of the elastomer latex particles due to the aforesaid ion exchange phenomenon between the cationically charged elastomer particles in the latex with cation exchangeable ions within the galleries of the stacked platelets of the water-swellable clay to cause an intercalation/exfoliation of the clay, thereby causing the elastomer particles to coagulate. The relatively bulky cationically charged (positively charged) coagulated elastomer particles, in turn, enter the galleries of the negatively charged stacked platelets to further expand the distance between the platelets and further promote a bonding of the elastomer particles to the surface of the platelets which consequently results in a more complete exfoliation (delamination) of the platelets.
Moreover, by operation of this invention, a relatively hydrophilic water-swellable clay is substantially converted to more hydrophobic exfoliated platelets, which may include intercalated stacks of platelets, which are more compatible with diene-based elastomers and therefore more suitable for dispersion therein the as particulate reinforcement therefor.
The nanocomposites of this invention may also be used to create rubber composites as blends of such nanocomposites with other elastomers, ingredients and/or coupling agents which may be used as components of articles of manufacture, including tires. Thus such articles of manufactures may be composed of a rubber composition comprised of said nanocomposite and/or said rubber composite.
Indeed, while some aspects of the process might appear to be somewhat simplistic in operational nature, it is considered herein that the overall technical procedural application is novel, a departure from past practice and inventive. This is particularly considered herein to be true where nanocomposites in a form of exfoliated, hydrophobic, platelet reinforced elastomer compositions are desired for use in composites of articles of manufacture such as components for tires.
In practice, the complete exfoliation of the polymer-bound platelets can be determined, for example, by wide angle X-ray diffraction (WAXD) as evidenced by a substantial absence of an X-ray peak. Information concerning the intermediate partially exfoliated/intercalated state within the elastomer matrix can be qualitatively obtained by observing WAXD peak intensities and changes (increase) in the basal plane spacing between platelets.
It is appreciated that preparation of thermoplastic polymer/clay nanocomposites has heretofore been reported in Synthesis and Characterization of PMMA Nanocomposites by Suspension and Emulsion Polymerization by Haung and Brittain, Macromolecules 2001, 34, 3255 through 3260, published on the Web on Apr. 13, 2001. There, it has been suggested to introduce a smectite clay, which is said to be composed of silicate layers, into a pre-formed thermoplastic polymer latex such as, for example a poly(methyl methacrylate), or PMMA, latex, which contains a cationic surfactant, which may be a polymerizable surfactant, and which relies upon an interaction of the cations of the surfactant with anionic charges on the clay platelets and to which polymers of the polymerizable surfactant may become tethered to the surface of the platelets. It is understood herein, from the publication, that exfoliation of the platelets is obtained upon melt processing of the thermoplastic based nanocomposite. A significant purpose in the preparation of such a thermoplastic nanocomposite polymer of methyl methacrylate is understood to be enhanced heat durability of the poly(methylmethacrylate) thermoplastic polymer without a sacrifice in its clarity.
It is important to appreciate, however, that a significant aspect of this invention (to avoid confusion with the above referenced Brittain system) is that a multilayered water swellable clay which contains cationically exchangeable ions within the galleries between its layers (e.g. a smectite clay) is intercalated and at least partially, and preferably substantially, exfoliated in the presence of cationic elastomer latex to the exclusion of a thermoplastic polymer latex, and to the exclusion of a latex which contains an anionic surfactant, where an enhanced utility in the reinforcement of elastomer compositions is desired to promote enhancement of, for example, one or more elastomeric physical properties such as ultimate tensile strength, modulus (e.g. 300 percent modulus) and/or abrasion resistance properties of a vulcanized elastomer composition, particularly for components of articles of manufacture such as tires and particularly for rubber tire treads.
Multi-layered, stacked clay particles, (e.g. montmorillonite clay) have also reported in U.S. Pat. No. 5,883,173 in which a latex is provided comprised of water, surfactant, and layered clay having an interlayer separation and a cationic exchange capacity, wherein layered clay is intercalated by in situ polymerization of monomers selected from, for example, styrene and butadiene to thereby expand the interlayer separation of layered clay. It is not seen that such patent is directed to any spontaneous coagulation of a pre-formed latex via a cation exchange process.
In the practice of this invention, (to avoid confusion of said U.S. Pat. No. 5,883,173) and contrary to such patent teaching, a nanocomposite of an elastomer matrix and a dispersion therein of exfoliated platelets is required to be prepared by an ion exchange between ion exchangeable ions in the galleries of stacked platelets of a water-swellable clay and cations contained on pre-formed elastomer particles in an elastomer latex in which the pre-formed latex is spontaneously coagulated.
In particular, for this invention the water-swellable clay is introduced to said latex as a water dispersion of the clay which does not contain an intercalant for the clay so that the clay is not pre-intercalated with an intercalant prior to addition to the latex. The clay, of course, contains cation exchangeable ions within the galleries between the stacked platelets of the clay which are somewhat swollen by the water in which the clay is dispersed prior to its addition to the latex. The latex itself, to which the water dispersion of the clay is added, is required to contain cationically charged elastomer particles which are available for an ion exchange with said cation exchangeable ions in the said galleries within the clay.
In practice, it is, in general, not intended that laticies of functionalized elastomer particles which are functionalized by containing carboxylic acid groups or aldehyde groups or epoxide groups on diene-based elastomers are used in this instant invention.
For the purposes of this instant invention, the latex of positively charged elastomer particles (cationically charged elastomer particles) is procedurally prepared by free radical emulsion polymerization using:
(A) a non-polymerizable cationic surfactant, and/or
(B) a polymerizable cationic surfactant.
Optionally, additional cationic charge may be incorporated onto the cationic elastomer latex particles, during the polymerization of the monomers to form the elastomer latex particles, through the use and in the presence of:
(C) a polymerizable, non-surfactant, comonomer bearing a cationic charge,
(D) a free radical generating polymerization initiator bearing a cationic charge, and/or
(E) a free radical chain transfer agent bearing a cationic charge.
The case of practicing this invention by preparing a nanocomposite by blending a water/water-swellable clay dispersion with a latex prepared by procedure (A) above, it is considered herein that a partial exfoliation of platelets occurs, dependant somewhat upon a degree of migration of relatively bulky, coagulated elastomer particles into the galleries between the platelets of the intercalated clay (the clay being intercalated by the aforesaid ion exchange between the cationic ions of the cationic surfactant and the cation exchangeable ions in the said galleries).
The case of practicing this invention by preparing a nanocomposite by blending a water/water-swellable clay dispersion with a latex prepared by the procedure (B) above, it is considered herein that a more substantial exfoliation of individual platelets occurs in which the cationic surfactant portion of the latex, which is polymer-bound to the coagulated elastomer particles, ion exchanges with the cation exchangeable ions within the clay platelet galleries and therefore causes the cationically charged coagulated elastomer particles to enter the galleries and to which the relatively bulky elastomer particles become polymer-bound to the surfaces of the positively charged surfaces of the clay platelets which, in turn causes the galleries to expand and to promote a substantial exfoliation (delamination) of the platelets from the clay as a dispersion thereof into the polymer matrix itself.
The case of practicing this invention by preparing a nanocomposite by blending a water/water-swellable clay dispersion with a latex prepared by optional procedure (C) or by optional procedure (D) above, it is considered herein that a more substantial exfoliation of individual platelets occurs, as compared to procedure A or B, in which the cationic, free radical generating polymerization agent creates cations on the surface of the resultant elastomer particles in the elastomer latex. The resultant cationically charged elastomer particles in the latex ion exchange with the cation exchangeable ions within the clay platelet galleries so that the cationically charged coagulated elastomer particles enter the galleries in a manner that the relatively bulky, elastomer particles become polymer-bound to the surfaces of the positively charged surfaces of the clay platelets which, in turn causes the galleries to expand and to promote a substantial exfoliation (delamination) of the platelets from the clay as a dispersion thereof into the polymer matrix itself.
The case of practicing this invention by preparing a nanocomposite by blending a water/water-swellable clay dispersion with a latex prepared by optional procedure (E) above, it is considered herein that a more substantial exfoliation of individual platelets occurs in which the cationically charged chain transfer agent (e.g. 2-aminophenyldisulfide dihydrochloride) is introduced to the free radical polymerization. The chain transfer agent thereby creates cations on the surface of the resultant elastomer particles in the elastomer latex. The resultant cationically charged elastomer particles in the latex ion exchange with the cation exchangeable ions within the clay platelet galleries so that the cationically charged coagulated elastomer particles enter the galleries in a manner that the relatively bulky, elastomer particles become polymer-bound to the surfaces of the positively charged surfaces of the clay platelets which, in turn causes the galleries to expand and to promote a substantial exfoliation (delamination) of the platelets from the clay as a dispersion thereof into the polymer matrix itself
Water-swellable clays considered for use in this invention which are clays composed of a plurality of stacked platelets (e.g. very thin silicate based platelets) which contain cationically exchangeable ions in the galleries between such platelets. Representative of such clays are water swellable smectite clays, vermiculite based clays and mica based clays. Preferably such water-swellable clays are smectite clays. Representative of smectite clays are, for example, montmorillonite, hectorite, nontrite, beidellite, volkonskoite, saponite, sauconite, sobockite, sterensite, and sinfordite clays of which montmorillonite and hectorite clays are preferred. For various exemplary smectite clays, see for example U.S. Pat. No. 5,552,469. Such cationically exchangeable ions contained in such galleries are typically comprised of at least one of sodium ions and potassium ions, which may include calcium ions and/or magnesium ions, although it is understood that additional cationically exchangeable ions may be present. Typically, montmorillonite clay is preferred which contains sodium ions in such galleries, although it is understood that a minor amount of additional cationically exchangeable ions may be contained in such galleries such as for example, calcium ions.
It is to be appreciated that, in practice, emulsion polymerization derived elastomeric styrene/butadiene copolymers may be typically prepared, for example, by polymerizing the styrene and 1,3-butadiene monomers in a water emulsion medium via a free radical and redox polymerization initiators in the presence of an anionic surfactant. Representative examples of anionic surfactants may be found, for example, in McCutcheon's, Volume 1, “Emulsifiers & Detergents”, North American Edition, 2001, Pages 291 and 292, with representative examples on non-ionic surfactants shown on Pages 294 through 300 and examples of cationic surfactants shown on Pages 300 and 301.
For the practice of this invention, anionic surfactants are to be excluded.
However, if desired, a minor amount of a nonionic surfactant might be used (e.g. from zero to about 20, alternately about 0.1 to about 20 weight percent) of non-ionic surfactant based on the total of surfactants used.
Accordingly, for the practice of this invention, a cationic elastomer latex of, for example elastomers derived from suitable monomers to yield elastomers such as, for example, styrene/butadiene rubber, cis 1,4-polybutadiene rubber and/or butadiene/acrylonitrile rubber, is required to be made as a result of free radically polymerizing the elastomer precursor monomers such as, for example, a combination of styrene and 1,3-butadiene monomers, 1,3-butadiene individually, or a combination of 1,3-butadiene and acrylonitrile. Preferably, the elastomer is a styrene/butadiene elastomer.
By requiring an aqueous dispersion of a water-swellable clay (e.g. smectite clay) which does not contain an intercalant (e.g. does not contain a quaternary ammonium salt), in a water-swollen state where the galleries between platelets are expanded somewhat by being swollen with water, to be blended with a pre-formed cationic latex of the elastomer particles to cause an ion exchange to occur between the cationic latex and the cationically ion exchangeable ions within the swollen galleries of the clay (e.g. smectite clay), the practice of this invention is significantly different from:
(A) simply requiring a polymerization of the respective monomers to occur in the presence of a smectite clay, whether or not the latex itself is a cationic latex and
(B) simply coagulating the elastomer from an elastomer latex by a typical salt/acid elastomer coagulation method.
Therefore, for this invention, the smectite clay is intercalated and exfoliated, preferably substantially exfoliated, into platelets, preferably polymer-bound platelets) within the elastomer
(A) after the elastomer is pre-formed by polymerization of monomers such as, for example, styrene and 1,3-butadiene, in a water based medium to form a cationic elastomer latex, and
(B) prior to, or simultaneously with, the coagulation of the elastomer from the cationic latex.
Therefore, the water-swellable clay (e.g. smectite clay) is
(A) not intercalated during the polymerization of the monomers,
(B) not intercalated by physically blending the smectite clay with the elastomer after it has been coagulated and recovered as a dry elastomer and
(C) not intercalated by blending a smectite clay which has been pre-intercalated by treatment with a quaternary ammonium salt prior to blending the pre-intercalated clay with the elastomer.
As hereinbefore discussed, it is an important aspect of the invention that the cationic nature of the surface of the elastomer particles stabilize the elastomer particles in the latex and keep the elastomer particles from coagulating. However, by operation of this invention, the cations undergo an ion exchange with cation exchangeable ions (e.g. sodium ions) within the galleries between the platelets of the smectite clay to cause
(A) the cations to be withdrawn from the presence of the elastomer particles (in the case of the cation surfactant) and to thereby destabilize the latex (causing the elastomer particles to coagulate) and
(B) substantially essentially simultaneously (during the coagulation process of the elastomer particles) expand the distance between the plates of the clay to form expanded galleries (by exchanging the ions such as sodium ions in the galleries between the stacked platelets of the clay with the more bulky cations from the cationic surfactant) and therefore intercalate and exfoliate the clay and (c) allowing the destabilized elastomer particles to coagulate.
In this manner, then, the method of creating a nanocomposite as a dispersion of intercalated, preferably substantially exfoliated, platelets, preferably polymer-bound platelets, of the clay (e.g. smectite clay) within an elastomer host is considered herein to be novel and a departure from past practice and, moreover, it is considered herein that the resulting nanocomposite of the elastomer and dispersion of substantially exfoliated polymer-bound platelets prepared by such process is also novel and a departure from past practice.
Historically, blending of an organoclay with a thermoplastic or thermosetting polymer by a melt blending process is discussed in U.S. Pat. Nos. 4,739,007; 4,810,734; 5,385,776; 5,578,672 and 5,840,796. Historically, blending of an adduct of a mineral filler such as, for example, a montmorillonite clay, and a quaternary ammonium salt with at least one rubber and an organosilane is discussed in U.S. Pat. No. 4,431,755.
In one aspect, a montmorillonite clay, for use in this invention, might be described, for example, as a naturally occurring clay of a structure which is composed of a plurality of stacked, thin and relatively flat, layers, where such individual layers may be of a structure viewed as being composed of very thin octahedral shaped alumina layer sandwiched between two very thin tetrahedrally shaped silica layers to form an aluminosilicate structure. Generally, for such aluminosilicate structure in the naturally occurring montmorillonite clay, some of the aluminum cations (Al+3) are viewed as having been replaced by magnesium cations (Mg+2) which results in a net negative charge to the platelet layers of the clay structure. Such negative charge is viewed as being balanced in the naturally occurring clay with hydrated sodium, lithium, magnesium, calcium and/or potassium cations, usually primarily sodium ions, within the spacing (sometimes referred to as “galleries”) between the aforesaid aluminosilicate layers, or platelets.
In the description of this invention, the term “phr” is used to designate parts by weight of a material per 100 parts by weight of elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “vulcanized” and “cured” may be used interchangeably, as well as “unvulcanized” or “uncured”, unless otherwise indicated.